1. Field of the Invention
The invention relates to an automatic thread wiper for a sewing machine with a pneumatic/automatic presser foot lifter, with an air hose attached to the exhaust of the presser foot lifting cylinder directing the air flow to an air jet attached to the presser foot shaft behind the needle. When lowering the presser foot of such a device, the exhaust of the presser foot automatic cylinder is utilized to blow the end of the cut needle thread onto the top of the presser foot. When starting to sew again, the needle thread is pulled through the needle plate of the sewing machine.
2. Description of the Prior Art
For example from U.S. Pat. No. 3,614,935 and German Offenlegungsschrift 1,918,608, a device is known where the needle thread end is removed from underneath the presser foot on a sewing machine which is equipped with a thread cutting device. With such a device, the air jet flow is released in the immediate vicinity of the needle at the presser foot by a slanted bore drilled through the presser foot and directed to the underside of the presser foot, just before the presser foot is lowered completely onto the sewing material. The air jet is of such intensity that the thread end is blown upwardly from underneath the presser foot. According to the German publication the compressed air feeding channel also could open above the presser foot or needle bar.
The advantage of these systems is that the air pressure source is already available on machines with pneumatic presser foot devices. This German publication mentions that the air tube which is mounted close to the needle is attached to this air pressure source. The air jet generation has to be coordinated with the lowering of the presser foot movement of the piston rod of the presser foot cylinder, since the end of the thread has to be removed from the area underneath the presser foot sole before the presser foot is placed on top of the sewing material. This publication mentions advantages with the securing of machines which have pneumatic actuation devices for the presser foot. This publication mentions that the jet is suitably synchronized with the downward stroke, that it is synchronized with the compressed air movement in the pressure cylinder which causes the movement of the presser foot. No further teaching is provided.
Another device is known from U.S. Pat. No. 3,371,632, which device is also equipped with a pneumatic thread cutting device. The thread wiper consists of an air conduit, a manually activated needle valve and another air conduit which is attached to a rigid angle arm. This angle arm on its free end has two converging jet arms connected with fork-shaped jet-pipe pieces. The jet arms are directed toward the needle, diagonally downwardly towards the top of the material clamp plates from two sides, and the jet bounces off the clamp plates upwardly to raise the thread end, which is blown away upwards from the area underneath the clamp plates before placing the latter on top of the sewing material.
Another device is known from German Offenlegungsschrift No. 26 13 728. This device severs the thread chain from the stitching area of the material to be sewn on the machine, and is provided with a channel to dispose of the thread chain, an entrance to lead the thread chain into the channel, and a fixed and flexible knife in the area of the entrance connected with a piston of a pneumatic pulsator.
The above-mentioned devices are expensive requiring substantial modification either in the presser foot or in the particular formation of the conduits and its support. Furthermore none of these patents specify the necessary control for obtaining the jet flow at the optimum time during the lowering of the pressure foot, which also requires special design and expense associated therewith.
Conventional machines mount the solenoid valves and actuators for lifting the presser foot underneath the bed plate, which are separated in several units.
Accordingly it is an object of the present invention to improve the devices of the prior art and to provide an efficient device with as little expense as possible.